1. Next won the war in the west, big time. Very very big time. Whenever people talk about new React apps they inadvertently mean Next. On the Vue side Nuxt is the default winner, and Nuxt is just the Next of Vue. That means that by default, by reflexive instinct, people are choosing Next and MPA as their strategy. If you want to correct the overly extreme pendulum motion then you should be telling people to try out SPA. The last 8 years has been a feverish push for MPA. Even the Facebook docs point straight to Next, totally deprecating Create React App. This is also Facebook ceding their battle to Next.
2. Whenever people complain about the complexity of Next, they are complaining about the difficulties of cutting edge MPA strategy, which evolves on a year to year basis. SPA on the other hand is a story that has frozen in time for who knows how many years. Almost a decade?
3. Doing MPA is strictly harder than doing SPA, much much harder. You have to observe the server/client distinction much more closely, as the same page may be split down the middle in terms of server/client rendered.
4. If you're writing an SPA and want to be more like MPA and load data at the time of hitting a user-navigable endpoint, that's on you, benefits and costs and all. You can also load data with anticipation so the client navigation is basically instant.
5. For every sexy SEO-able front-facing property you're going to have many teams with internal apps or dashboards backing that front-facing property. That's where many React devs are employed. Do not unnecessarily take on burden because you're so eager to ship the first "frame" of your app in a perfect way.